The removal of particulates from a gas stream has long been a practice in a variety of industrial fields. Conventional means for filtering particulates and the like from gas streams include, but are not limited to, filter bags, filter tubes and filter cartridges. For convenience herein, the term "filter element" will be used to refer collectively to these types of filtration means.
Conventional filtration techniques utilize the filter element to stop particles through the depth of the element, and as the particles build up in and/or on the element, the filtration efficiency of the element is increased. After an amount of dust has caked on the surface of the filter element, the flow rate of gas through the element is reduced to a level where the bulk dust cake must be removed from the element, typically by some form of agitation, such as vibration or the like.
Filter elements are typically constructed from felts and/or fabrics made from a variety of materials, including polyesters, polypropylenes, aramids, glasses and fluoropolymers. Selection of the type of material used is typically based on the gas stream with which the filter element comes in contact, the operating conditions of the system and the type of particulate being filtered.
Polytetrafluoroethylene (PTFE) has demonstrated utility in many areas. As an industrial material, such as a filtration material, for example, PTFE has exhibited excellent utility in harsh chemical environments, which normally degrade many conventional metals and polymeric materials. PTFE is also usable over a broad temperature range, from as high as 260.degree. C. to as low as near -273.degree. C.
However, conventional non-porous PTFE materials possess insufficient porosity to be effective as filtration means, particularly in the case of unexpanded PTFE in sheet form. Alternative means have been developed, such as the formation of woven felts or mats of unexpanded PTFE fibers, whereby particles are trapped between the fibers in the weave. Limitations still exist in these materials, however, due at least in part to the non-porous nature of the PTFE.
A significant development in the area of particle filtration was achieved when expanded PTFE membrane was incorporated as a surface laminate on conventional filter elements. One example is taught in U.S. Pat. No. 4,878,930, directed to a filter cartridge for removing particles of dust from a stream of moving gas or air. Preferred filter media for the cartridge are felt or fabric composites containing a layer of porous expanded polytetrafluoroethylene membrane.
Use of the expanded PTFE membrane greatly enhanced the performance of filter elements because the particles collected on the surface of the expanded PTFE, rather than in the depth of the elements as was occurring in the absence of the expanded PTFE layer. Several significant advantages were observed with these filter elements. First, the filtration efficiency of the elements was high immediately from the outset of the filtration process, and it was not necessary to "build up" a cake of particles to achieve high efficiency. Second, the elements lasted longer because particles were not getting into the backing fabric of the element and rubbing on the fibers to wear them out. Third, the cleaning energy needed to clean the particle cakes off of the elements was lower because the surface of the membrane was smooth and had a lower surface energy.
A filter bag made completely of expanded PTFE is described in U.S. Pat. No. 4,983,434, which shows an expanded PTFE membrane laminated to a felt of carded expanded PTFE staple fiber. This filter bag provides good pulse jet cleaning capabilities due to the strength and flexibility of the expanded PTFE, while also providing good heat resistance, chemical inertness and high air permeability.
In each of the cases described above incorporating expanded PTFE, the filter element comprises a membrane laminated to a backing material which purportedly provides support to the membrane to permit it to withstand the rigors of the filtration and cleaning processing. Conventional teachings in the field of filtration focused on the need for heavier support, or backing, materials to provide more durability to the filter element; however, the use of heavier support materials for higher strength and durability led to a trade-off with blocking more airflow through the filter and requiring more energy to clean the filter element.
For example, laminates weighing up to 22 ounces/square yard (745 g/square meter) were developed which provided longer life, but were heavy, bulky and required more energy to flex or clean the elements. Further drawbacks to such materials included, but were not limited to, high manufacturing costs due to the complex nature of the laminated elements, wear of the bags due to internal stresses between the laminated layers, the need for precision fitting of the elements in the filter assemblies in order to prevent movement of the filter element against the support, resulting in wear, and eventually failure, of the bag, difficulty in achieving effective cleaning, contamination due to particulation of the laminated media, larger quantities of material to dispose of after the filter bags wore out, and the need to accommodate the excess bulk of the laminated filter elements within the design of the filtration assemblies.
The novel filter elements of the present invention are designed to solve these problems and provide significant advantages over the filter elements of the prior art, as described in more detail herein.